Glass-ceramic articles, i.e., precursor glass bodies which have been subjected to a controlled heat treatment in order to effect crystallization, are well known in the art. Glass- ceramics which have as a dominant crystal phase, that of canasite, were disclosed in U.S. Pat. No. 4,386,162 (Beall). That canasite patent disclosed translucent-to-opaque glass-ceramic articles with a composition consisting essentially, in weight percent on the oxide basis, of about 45-75% SiO.sub.2, 8-30% CaO, 2.5-12% F, 3-15% Na.sub.2 O, 0-20% K.sub.2 O, 5-25% Na.sub.2 O+K.sub.2 O, 0-6% B.sub.2 O.sub.3, 0-7% A and 0-12% ZrO.sub.2. The most preferred composition was stated to have an analyzed composition of about 50-65% SiO.sub.2, 0-3% B.sub.2 O.sub.3, 0-3% Al.sub.2 O.sub.3, 15-24% CaO, 3-13% Na.sub.2 O, 3-15% K.sub.2 O, 11-22% Na.sub.2 O+K.sub.2 O, and 0-8 % ZrO.sub.2.
Generally, in the absence of added colorants, opaque glass-ceramic articles, such as the above canasite, display a white appearance. Corning Code 9608 glass-ceramic, marketed by Corning Incorporated, Corning, N.Y. under the trademark CORNING WARE.RTM. is another example of a glass-ceramic which appears opaque and creamy-white in its uncolored state.
The soft translucence of the basic white, opaque canasite glass-ceramic, perceived by architects and designers as a having marble-like "depth", serves as a key advantage of this material over many other architectural materials. However, for any architectural application, having a wide range of available strong bright colors in addition to white would be highly desirable. Masking canasite's translucence with an opaque colored glaze, as is done with most commercial floor tiles, would nullify the uniqueness of this material. Placing the color in the body, however, and glazing if desired with a clear transparent glaze, preserves much of the illusion of "depth", while satisfying the need for a range of bright colors. Additionaly, another advantage of this approach is that a clear glaze on a colored body, such as a tile, provides uniformity of color despite variations in glaze thickness and despite any scratches or chips in the tile.
It is thus highly desirable to produce bright and vividly colored opaque glass-ceramics, and therefore it is one of the main objectives of this disclosure to disclose glass ceramics possessing this characteristic.
The coloration of glass-ceramics is well known in the art and thus the subject of numerous patents. Typical colorants, known in the glass art, have been incorporated into a base precursor glass compositions, and then crystallized in situ to form colored glass-ceramic articles. However, the range and brightness of the colors capable of being produced in opaque glass-ceramics is limited. For example, U.S. Pat. No. 4,192,688 (Babcock et al.) discloses colored opaque and/or transparent, low expansion, crystallized glass-ceramics formed from different colored crystallizable base glasses and having lithium-containing crystals as the predominant crystalline species. The base glass compositions consisted of at least the following essential components in the following weight percentage limits, based on the total glass composition:
SiO.sub.2 50-75 PA0 Al.sub.2 O.sub.3 16-35 PA0 Li.sub.2 O 3-5.5 PA0 Colorants 0.005-2 PA0 Nucleating Agent Variable
Li.sub.2 O+Nucleating Agent At least 5.5
The colors obtained in the opaque glass-ceramics described in this reference are limited to the typical softer pastel and pale earth tones, e.g., light blues, greens, grays and yellows. Therefore, to produce a transparent glass-ceramic in which somewhat brighter colors can be obtained, this reference teaches the strict and careful control of the crystallization through the use of a nucleating agent.